1. Field of the Invention
The present invention generally relates to a fluid-filled elastic mount adapted to provide a vibration damping effect on the basis of flows or resonance of a fluid which is forced to flow through an orifice passage. More particularly, the present invention is concerned with such a fluid-filled elastic mount which is capable of changing damping characteristics by controlling fluid communication of the orifice passage or passages with fluid chambers.
2. Discussion of the Prior Art
As one type of a vibration damper interposed between two members of a vibration system, there is known a fluid-filled elastic mount as disclosed in JP-A-60-104824, wherein a first and a second mounting member which are attached to the respective two members of the vibration system in a vibration damping manner are spaced apart from each other by a suitable distance and are elastically connected to each other by an elastic body. The elastic mount further includes a partition member which is supported by the second mounting member. A pressure-receiving chamber which is partially defined by the elastic body is formed on one side of the partition member, while a variable-volume equilibrium chamber which is partially defined by a flexible diaphragm is formed on the other side of the partition member. These pressure-receiving and equilibrium chambers are filled with a suitable non-compressible fluid, and communicate with each other by an orifice passage. A pressure of the fluid in the pressure-receiving chamber changes due to elastic deformation of the elastic body upon application of vibrations to the elastic mount. This type of the elastic mount provides a high damping or isolating effect based on resonance of the fluid flowing through the orifice passage, which effect cannot be provided by an elastic mount that relies upon only the elasticity of the elastic body for damping the input vibrations. For this reason, the elastic mount of this type is favorably used as an engine mount for a motor vehicle, for example.
Generally, a fluid-filled elastic mount when used as a vehicle engine mount is required to exhibit different damping or isolating characteristics depending on the type of the input vibrations having different frequencies and amplitudes. However, the range of frequency of the input vibrations that can be effectively damped by fluid flows through an orifice passage is relatively narrow. Therefore, a fluid-filled elastic mount using a single orifice passage is not generally capable of exhibiting desired damping characteristics to a satisfactory extent.
Another type of the fluid-filled elastic mount is disclosed in JP-A-60-169323. Described in detail, the elastic mount has a support rod which is secured at one axial end thereof to a central portion of the flexible diaphragm, and supports a valve element at the other axial end, so that the valve element is opposed to an opening of the orifice passage. The diaphragm is covered with a cover member so as to define an air-tight operating chamber therebetween. In the thus formed operating chamber, there is disposed a coil spring which downwardly biases the support rod such that the valve element provided at the other axial end of the support rod closes the opening of the orifice passage. In the meantime, when the operating chamber is connected to a suitable vacuum source, the support rod is retracted toward the cover member against the biasing force of the coil spring so that the valve element is released away from the opening of the orifice passage to permit fluid communication therethrough. In the thus constructed engine mount, the orifice passage is selectively controlled to be operative and inoperative for permitting and inhibiting the fluid communication therethrough, by the valve element as described above, to thereby adjust the vibration damping characteristics based on the fluid flows through the orifice passage.
In the elastic mount constructed as described above, the valve element or valve means needs to be disposed within the pressure-receiving chamber or equilibrium chamber, undesirably leading to a complicated structure, an increased cost of manufacture and lowered production efficiency of the elastic mount. Further, the elastic mount inevitably tends to be large-sized. In the above-constructed elastic mount, since the flexible diaphragm partially defines the operating chamber, the diaphragm is directly exposed to the reduced pressure in the operating chamber when the operating chamber is connected to the vacuum source for retracting the valve element away from the opening of the orifice passage. This arrangement results in considerable deterioration of a degree of elastic deformation of the diaphragm, i.e., deterioration of its capability to permit a volume change of the equilibrium chamber. Accordingly, the fluid is not likely to effectively flow through the orifice passage, whereby the elastic mount does not exhibit the intended vibration damping effect.
Another type of the fluid-filled elastic mount is disclosed in JP-A-59-93537 and JP-A-59-117929, wherein communication holes of the orifice passage for fluid communication with the equilibrium chamber are opposed to a central portion of the flexible diaphragm, while a bag-like elastic body is disposed adjacent to the central portion of the diaphragm on one of the opposite sides thereof remote from the equilibrium chamber. In this bag-like elastic body, there is formed an air-tight operating chamber. The bag-like elastic body is displaced in opposite directions toward and away from the diaphragm, by selectively supplying and discharging a pressurized medium (hydraulic pressure) to and from the operating chamber, so that the diaphragm is pressed onto or retracted from the communication holes for closing or opening the same. Thus, the orifice passage is selectively controlled to be operative or inoperative for permitting or inhibiting the fluid communication therethrough.
In the above type of the elastic mount, it is required to supply the pressurized medium having a relatively high positive pressure to the operating chamber, so as to obtain a sufficiently large amount of force for pressing the diaphragm onto the communication holes to close the same. In general, in the motor vehicle equipped with an internal combustion engine, it is rather difficult to obtain compressed air having a positive pressure which is higher than the atmospheric pressure, while it is rather easy to obtain a negative or reduced air pressure which is lower than the atmospheric pressure. In other words, it is difficult to obtain a sufficient amount of compressed air having a positive pressure to be supplied to the operating chamber for pressing the diaphragm onto the communication holes so as to close the same, against a pressure of the fluid flowing through the orifice passage. In this case, the orifice passage may not be fully closed with high stability. When a relatively large positive pressure is applied to the operating chamber, the wall of the operating chamber may be outwardly expanded or deformed, adversely influencing the degree of displacement of the diaphragm, i.e., its capability to permit a volume change of the equilibrium chamber. Accordingly, the elastic mount may not exhibit the intended vibration damping or isolating characteristics.
In an attempt to effectively obtain the pressing force for pressing the diaphragm onto the communication holes, it is considered possible to utilize the elasticity of the bag-like elastic body, by disposing the elastic body in a compressed state, for instance. In case where the bag-like elastic body has a wall thickness large enough to exhibit effective elasticity for pressing the diaphragm, the bag-like elastic body would not likely to be easily deformed upon application of the reduced pressure to the operating chamber formed in the elastic body. In this case, the communication holes may not be fully opened for the fluid communication through the orifice passage. When the bag-like elastic body is kept in the compressed state, the pressing force provided by the elasticity of the bag-like elastic body would be undesirably lowered in a relatively short period due to creep of the rubber material of the elastic body. Accordingly, the orifice passage would not be adequately controlled to be selectively operative or inoperative for permitting or inhibiting the fluid communication therethrough.